Dendritic cells (DCs) are key immune sentinels that link innate recognition of pathogens to adaptive immune responses. DCs comprise two main evolutionarily conserved subsets, type I interferon- producing plasmacytoid DCs (pDCs) and antigen-presenting classical DCs (cDCs). The two DC subsets share common dependence on the cytokine Flt3 ligand, yet their cellular and molecular properties are very distinct. It is currently believed that cDCs and pDCs develop from a common DC progenitor (CDP) in the bone marrow. However, some recent evidence is not compatible with this model, warranting an unbiased re-examination of DC development using lineage tracing in vivo. We will use transgenic mouse strains expressing a tamoxifen-inducible Cre recombinase (CreER) to label stem/progenitor populations and trace their differentiation into DCs. Specifically, we will examine the kinetics and sequence of hematopoietic stem cell differentiation into DC subsets (Aim 1) and the differentiation potential of CDPs (Aim 2). These studies would generate an updated model of DC development based on in vivo lineage tracing. They would also generate novel strains for inducible Cre recombination with utility for lineage tracing and gene targeting.